Do install "github.com/hashicorp/consul/vendor" into dev package

This unbreaks autopkgtests, cf. LP: #1940152.

Ideally we would not install vendored dependencies here and instead have all dependencies packaged seperately. Right now, the autopkgtests indicate that the lack of the "consul/vault/api" package prevents dependant packages from building correctly. A better fix would be to update the src:vault package to version 1.0.5 or later and expose that golang library. When this is the case, this commit can be reverted.